Frequency modulation system



Feb. 29, 1944;. G L, USSELMAN 2,343,099

FREQUENCY MODULAT ION SYSTEM Filed Sept. 9 1942 2 Sheets-Sheet 1 l All I vvv o 0 o o". 0 0 0 0 j 6 I .ea'

INVENTOR GEORGE L. USSELMAN SIGNAL SOURCE I ATTORNEY Feb. 29,1944.

G. L. USSELMAN 2,343,099

FREQUENCY MODULATION SYSTEM Fil'ed Sept. 9, 1942 2 Shee't-Sheet .2

- OUTPUT ll b F/JI \ao c1 L1 *1 I SIGNAL SOURCE INVENTOR 7 GEORG USSELMAN- A'I'I'ORNEY Patented Feb. 29, 1944 TED Y PA EN I 1 4 9 .1 FREQUENCY MODULATION SYSTEM or to Radio Corporatio tion of Delaware 11 of America, a. corpora- Application September 9, 1942, Serial No. 457,702

7 Claims. (01. lie-171.5)

This application concerns a new and-improved method of and means for producing wave energy and modulating the frequency thereof inyaccordance with control waves such as for example voice signals, telegraphy signals, space wave keying signals. etc.

This application is a continuation-in-part of my United States applications Serial No. 338,837, filed June 5, 1940, now U. 5. Patent #2,298,436, dated October 13, 1942, and Serial No. 361,506, filedOctober 17, 1940, now U. S. Patent #2298; 438, dated October 13, 1942.

In the present application I have shown several modifications of the system comprising the oscillator, the modulator, and the connections between the oscillator and the modulator tube. The modulation system of this application also involves modified phase shifting means in the circuits coupling the modulator to the generator.

In describing my invention reference will be made to the attached drawings wherein:

Figs. 1 to 3 inclusive e'ach illustrates a different modification of,-my system. Each modification comprises an oscillation generator the frequency of oscillation of which is stabilized by a piezo-electric crystal or equivalent means, a tube device, excited by voltages developed in the oscillator, which is arranged to amplify the 'said voltages and feed them back in phase displaced relation to the oscillator. Means is also provided to modulate the said tube which, for convenience, has been designated as the modulator tube. In themodification of Figs. 2 and 3 the oscillator tube is also modulated and differentially with respect to the. modulation on the modulator tube to thereby balance out concommitant amplitude modulation.

Referring to Fig. 1 of the drawings, V l is an electron discharge device having a grid G3 grounded, grids GI and G2 connected in an oscillation producing circuit including the piezoelectric crystal. The grid G2 is coupled to ground by radio frequency by-passing and direct current blocking condenser 6 and to a source of chaging p0tentia1 by means of a resistor R4.

- The control grid G! is connected to one terminal of the piezo-electric crystal X, the other terminal of which is grounded' This grid-GI is also connected to ground through a potentiometer resistor RI and the cathode K is tapped to a point on RI. The anode ill of tube VI is connected to a tank circuit Cl, LI- and an output circuit may be coupled to the inductance Ll; this output circuit in Fig. 1, comprising a coupling inductance L2.

The modulator tube V2 in Fig. 1 has its cathode 20 grounded and connected to the negative terminal of a source of potential 25, the positive terminal of.which runs through'the secondary 'winding of a transformer T to the screen grid electrode 34. The control grid electrode 24 is connected to a parallel tuned phase adjusting circuit LC, one terminal of which is connected by a blocking condenser 26 to a point on resistor Rl to derive excitation voltage there? from for the grid 24. The anode electrode 40 is connected by a resistor R3 to asource of po-' tential not shown. A point on R3 is coupled by a blocking condenser 29 to the high radiofrequency potential terminal 'of the piezo-electric crystal X, and to the control grid GI. A direct-current circuit for the control grid 24 is completed by a resistance R2.

The crystal oscillator-circuit of Fig. 1 is of the grounded anode type in which the second grid G2 or tube Vi acts as the oscillator anode and it is grounded for radio frequency through a bypass condenser 6. The thirdv grid G3 is grounded directly as shown. The anode of tube Vi works into a tuned output circuit Cl, LI and is substantially only electronically coupled to the crystal oscillator circuit. The grounded grid G3 shields the output circuit with respect to the generating circuits. Grid leak resistor RI is also connected to'the control grid of tube VI and to ground.

The arrangement shown in Fig. 2. is somewhat similar to the arrangement shown in Fig. 1. In Fig. 2, however, the oscillator tank circuit LlCl, which is connected with the anode ill of tube VI, has the mid-point thereon connected to ground by a blocking condenser 3|. This mid-point connection'is also connected to the terminal of R3 remote from the anode 40 of tube V2 and also to a source of direct-current potential. In this modification, as in Fig. 1, the output is derived from a reactance L2 cou- In the modification shown in Fig. 2, a coupling capacity C2 is also connected between the grid GI and the anode ill of tube Vi and oscillation generation is produced by virtue of the anode to control grid coupling C2. Moreover, the excitation circuit for the grid 24 of tube V2 is coupled by phase adjuster LC and coupling condenser 26 to the high radio-frequency voltage terminal of the crystal X instead of to RI as in Fig. 1. Furthermore, differential modulating potentials are supplied by the transformer T to the screen VI. By this opposed modulation of the screen grids, amplitude modulation in the output circuit LI-CI is reduced.

In the system of Figs. 1 and 2 the circuit C, L must be slightly detuned with respect to the frequency of the generated waves to act as phase shifters. The action of these frequency modulator, circuits is as follows, assuming that proper steady potentials are applied. A small amount of excitation is picked up from the oscillator circuit, say of a phase like the oscillator grid GI excitation phase, by the tap on RI in Figs. 1 and 2. In passing through the phase shifters C, L, the phase of the excitation is retarded say 90 degrees. Of course, most phase shifter networks depend on the terminating resistance or impedance as one of the 'elements'in the mechanism of phase shifting or phase rotation, so that the excitation voltage reaching the control grid 24 of tube V2 is say lagging 90 degrees in phase behind thevoltage at the oscillator grid. In

passing through the tubeV2 the modulating excitation energy is amplifled and is reversed or rotated 180 degrees i phase. This modulating excitation energy is then applied to the control grid GI of oscillator t'ube VI through conductor D and the blocking condenser 29. The modulating excitation energy therefore leads the oscillator excitation energy by 90 degrees in phase. It should be stated that this phase angle can be made more or less than these assumed values by, adjustments of the phase shifters. In some cases this is desirable in order to change the degree of frequency modulation. The resultant excitation on the grid of tube VI is then leading the normal oscillator excitation.

When the tube V2 is modulated by the signal the amount of modulating excitation energy delivered to the grid of tube VI from the anode of tube V2 is also modulated. Consequently, addition of the two components of excitation on the grid of tube VI causes the phase angle of this excitation to swing between the limits of the two components. This, in turn, causes the frequency to be modulated in accordance with the signal oscillations.

If in the phase shifting circuit C, L the excitation energy is advanced in phase, by the same process or reasoning, the modulating excitation delivered to tube VI by tube V2 will be retarded in phase and the resultant excitation on the grid of tube VI will lag the normal oscillator phase. The frequency modulation in this case would be in opposite direction to that described in the first case.

It should be pointed out that the phase shift in the oscillator excitation occurs for each cycle, thus resulting in frequency shift or frequency modulation. This phase shift continues for each cycle until the frequency shift produces an equal and opposite phase shift in the circuits.

The modulator shown in Fig. 1 produces some amplitude modulation along with the frequency modulated carrier. However, the use of limiters in succeeding stages of the transmitter will eliminate the undesired amplitude modulation from the output. a

The frequency modulation circuit shown in Fig. 2 eliminates the amplitude modulation from the frequency modulated carrier output by modulating the amplitude of the output from tubes VI and V2 in opposite sense. In this modification changes in generated wave amplitude caused by modulating tube V2 are opposed and compensated by corresponding but opposed moducontrol grid GI of tube VI.

lations on the grid of VI. and bias point on the secondary winding of transformer T is properly chosen, substantially all amplitude modulation is eliminated from the output circuit in Fig. 2. This would also increase the efliciency of the frequency modulation.

I The modification in Fig. 3 consists of a crystal oscillator circuit and a modulator circuit as in the prior arrangements. The crystal oscillator circuit consists of a tube VI, tank circuit CI-,-LI connected with the anode I0, crystal X, grid resistor RI and feedback condenser C8. The cathode III of VI is grounded. The crystal X is connected between the control grid GI and the cathode III of tube VI. The grid leak RI parallels the crystal X. The feedback condenser C8 is connected between the anode I0 and the The center point of tank coil LI' is grounded by condenser 3| for radio-frequency and one end of the tank circuit LI, CI is connected to the anode III of tube VI. The modulator consists of tube V2, the phase shifter element LP, the grid resistor R2 and the anode resistor R3. The cathode 20 of tube V2 is grounded and the grid resistor R2 is connected between the control grid 24 and cathode 20 of tube V2. The anode 40 of tube V2 is connected to a source of positive potential through the center point of coil LI, and through resistor R3. The anode 4|! of tube V2 is also coupled to the grid of tube VI through a blocking condenser 26 and through a phase shifting coil LP. The grid 24 of tube V2 is coupled to the tank coil LI through a blocking condenser 30 as shown. The screen grids G2 and 34 of tubes VI and V2 are modulated in phase opposition from signal source A through transformer T. Assuming proper steady potentials applied to the cricuit of Fig. 3, the crystal oscillator circuit will oscillate by virtue of the feedback to the control grid from the anode of tube VI through condenser C8 and the piezo-electric action of the crystal X, grid bias being by the rectified grid current passing through resistor RI. The crystal tends to hold constant frequency for the oscillator. The control grid 24 of tube V2 is excited from tank circuit CI,.,LI through coupling condenser 30. This energy is amplified in the anode of tube V2 and fed back to the grid GI phase shifter LP causes the modulation excitation energy from tube V2 to lag say degrees. The resultant excitation on the grid GI of VI swings between the limits of zero to 90 degrees leading in the case assumed. If the grid 24 of V2 is tapped on the anode end of tank circuit CI, LI its excitation will 'be opposite in phase with respect to the excitation on the grid GI of VI because of the reversal in the tube VI. Then the reversal in tube V2 brings the amplified modulating excitation back in phase with the voltage on grid GI of tube VI. However, the phase shifter LP again causes a lag of 90 degrees, and the resultant excitation is lagging, which is opposite to the first case cited. The effective excitation on the grid GI of tube VI will, in either case, be the resultant of the oscillator VI excitation and the modulating excitation from the tube V2. Now, if the tube V2 is modulated in That is, u the ground amplitude the amount of modulating excitation delivered from V2 to the grid Gi of VI will also be modulated in amplitude. If the oscillator delivers steady excitation, the result is a varyingexcitation phase angle on the grid GI of VI. This causes the frequency of the'oscillator to vary. Since the modulating transformer circuit T modulates tubes VI and V2 in opposite sense a greater degree of phase change in the resulting radio-frequency voltage on the grid of V! is produced and, consequently, a greater degree of frequency modulation is obtained. If the ground and bias tapping point on the secondary of transformer T is properly chosen-any amplitude modulation in the output is balanced out, leaving only wave energy modulated in frequency in accordance with the signal oscillations from source A.

The phase-changing network LC may be of any desirable type. Figs. 1 and 2, or it may instead be a series coil, parallel condenser combination arrangement. It

may be coils or condenser arrangements with re sistors. It may consist of one stage or a many stage filter. It should also .be recognized that the terminating load on a filter,'in this case R2,

and the control grid capacity of tube V2 are part It may be made as shown in trodes for feeding said derived voltage back to said one of said electrodes whereat the generated voltage phase 'is of said certain phase, a reactance in one of said last two couplings-for shifting the phase of said last named voltage fed back to said one electrode to an extent such that the said fed back voltage and the said generated voltage are substantially in phase quadrature on said 4 one electrode, connections toan electrode in said of thecircuit .with the filter afiecting the change in excitation energy phase angle.

What is claimed is:

1.. In a wave length modulation system, an electron discharge devicehaving oscillation gener-' ating electrodes, an oscillation generating circuit, including a frequency determining piezo-e'lectric crystal, regeneratively coupled to said electrodes, the generated voltage on at least one of said electrodes being of a certain phase, connections for deriving voltage of the generated frequency from-said circuit and feeding said derived voltage back to said one of said electrodes whereat the generated voltage phase is of'said certain tube for modulating the amplitude of the fed back voltage to thereby correspondingly modulate the length of the oscillations generated and incidentally amplitude modulate the oscillations, and'connections to an electrode in said device for modulating the amplitude ofthe oscillations phase, a reactance for shifting the phase of said last named voltage-fed back to said one electrode to an extent such that the said fed back voltage and the said generated voltage are substantially in phase quadrature on said one electrode, connections for modulating the amplitude of the fed back voltage in accordance with signals to'thereby produce desired wave length modulation'of the generated oscillations and undesired amplitude modulation thereof, and additional connections for modulating the potential on an electrode of said discharge device in a direction to compensate said undesired amplitude modulation.

' 2. In a wave length modulation systejn, an electron discharge device having oscillation generating electrodes, an oscillation generating circuit, including a frequency controlling piezo-electric crystal, regeneratively coupled to said electrodes. the generated voltage on-at least one of said electrodes being of a certain phase, an electron -dis-.

from said circuitand feeding amplified derived voltage back to said one of said electrodes whereat the generated voltagephase is of said certain phase, a reactance for shifting the phase of said in a direction to compensate said incidental modulations.

4. In a wave length modulation system, an electron discharge device having oscillation generating electrodes, an oscillation generating circuit, including a frequency controlling piezoelectric crystal, regeneratively coupled to said electrodes, the generated voltage phase on at least one of said electrodes being of a certain phase, an output circuit coupled to said generating circuit, an electron discharge tube having input electrodes coupled to said output circuit for deriving voltage of the generated frequency from said circuit, said tube having output electrodes cou'pied'to said one electrode for feeding said derived voltage to said one of said electrodes whereat the generated voltage phase is of said certain phase, a reactance in one of the cou-. plings to said tube electrodes for shifting the phase of said last named voltage fed back'to said one electrode to an extent such that the said fed back voltage and the said generated voltage are substantially in phase quadrature on said one electrode, a source of signals for modulating the impedance of the tube to thereby modulate the amplitude of. the fed back voltage in accordance with signals, and connections between said source and device for modulating the impedance of the device in accordance with said signals.

5. In a wave generating and wave length modulation system an electron discharge device having'a cathode and a plurality of control electrodes, an oscillation generating circuit connected between said cathode and two of said control electrodes, one of'which operates in said oscillation generating'circuit as an anode, an output circuit connected with said output electrodes and coupled to said oscillation generation circuit, a piezo-electric crystal coupled between two of the electrodes in saidv oscillation generating circuit, a modulator tube having an anode, a cathode and a control electrode, an excitation circuit including phase shifting means coupling said oscillation generating circuit to the control electrode of said modulator tube, and means for modulating the impedances of said device and said tube in accordance with control potentials to thereby modulate the length of the oscillations generated.

6. In a wave generating and wave length modulation system, an electron discharge device having a cathode and a plurality of control electrodes, an oscillation circuit connected between said cathode and one of said control electrodes,

a piezo-electric crystal in said circuit, a reactive electrode of said tube in accordance with signals.

'7. In a wave generating and wave length modulation system, an electron discharge device having a cathode, an anode, and a plurality 01 control electrodes, an oscillation circuit comprising a piezo-electric crystal in a holder connected between said cathode and one of said control electrodes, a reactance coupling said anode to said one of said control electrodes to provide regeneratlon in said tube, a reactive output circuit coupledto said anode, a second tube having an anodeja cathode and a control electrode, a coupling between a point'on said reactive circuit and the control electrode of said second tube, a connection between one terminal of said piezo-electric crystal and theanode of said second tube. and means for modulating the impedances of said second named tube and said device in accordance with control potentials.

GEORGE L. USSELMAN. 

